British politics is a lot like the class system. You're supposed to know your place, and if you are a Liberal Democrat that place isn't meant to be breakfasting with the prime minister at Chequers. He's the first liberal leader for generations to mistake democracy for an invitation to help run the country. He's broken the code. He's slept with the wrong sort. He's even married them. And now he's being hated for it.

Hated most of all by Labour, which outsourced much of its morality to the Lib Dems while doing whatever was required to win power for itself. In this view of the natural order of things, the third party existed only as a dumping ground for guilt, a pool of moral purity, a party that could promise everything good and oppose everything bad while rarely getting in its superiors' way.

The big boys ran the country. The Lib Dems were supposed to yelp admonitions from the sidelines. Labour commissioned the Browne review on student finance. The Lib Dems' assumed duty was to oppose it, however dotty the grounds for doing so.

Then along came the coalition, and everything became extraordinarily different. The old world has been blown inside out. The people who never had power suddenly have lots of it, and those who assumed it was theirs to keep can only complain. To a political generation that enjoyed a comfortable maturity under Labour, it remains outrageously inappropriate to hear of Nick Clegg and Danny Alexander sitting with a Tory chancellor and prime minister at quadrilateral meetings deciding the spending review.

This mindset does not judge the coalition for its actions but condemns the fact that it exists. The fury – far beyond the scale of anything the Lib Dems expected – is rooted in a hostility to pluralism that regards Conservatism as something approaching an evil, and any Lib Dem association with it an unnatural compromise. Presumably, the only acceptable outcome would be ceaseless Labour rule.

These critics forget that we nearly had that, and it went wrong; that their time is done; that the country did not vote for their ideas any more than it voted, as a block, for anyone else's; that what seems to them to be the wrecking of all that is decent is to others the potential rectification of much that is wrong. They do not understand the great significance of the coalition's creation and success: that Britain may never again want to be ruled by one political gang against all the others.

Loathe this government if you will, but at least acknowledge that neither side in it got all it wanted at the election and that neither has sold out all of its principles. The strangeness of co-operation exposes its component parts to the easiest of attacks: of promising one thing before an election and doing another after it. But as Clegg has pointed out, the reason he is not implementing the Lib Dem manifesto is because the Lib Dems lost. So did everyone else.

Riled, Lib Dems are making a poor job of defending themselves. They are embarrassed to speak confidently – not so much because of the deal they did, better than anyone guessed before the election, but because they never presented themselves as deal-makers. Instead, they presented themselves as tellers of fantastical truths, signing pledges on tuition fees the leadership never thought they'd need to return to. That was the worst of the Lib Dems: indulging an unworkable policy that amounted to an unaffordable middle-class subsidy dressed up as principle.

Some of the voters won over by such things are angry. Many have decided to support Labour instead. Fair enough: many Lib Dem voters – and many members too – were content with the perfection of irrelevance. Clegg, though, is dealing with the imperfection of power. He's hoping to be judged on what he does: on his multibillion pupil premium; on being in a government brave enough to cut prison numbers and defence spending and middle-class benefits; on political reform. It hurts when everyone throws rocks at you – but it is better that the rocks come from all sides. It suggests the claim of balance is real. Navy admirals are angry, so is the Daily Express – and so are many Guardian readers.

The spending review has been the making of the coalition, and if it goes wrong it will be the destruction of it. Ministers from rival parties have been bound together in unthinkable alliances: Clegg backing Iain Duncan Smith on welfare; Eric Pickles seeking Lib Dem allies for localism. Of six sticking points thrashed out before the spending review was signed today, three went in the Lib Dem direction – but to some that very act of dealing is to be damned.

Britain's political tribes are determined as much by emotion and prejudice as any absolute sets of policy. There are instincts, ideas and loyalties that pull one way or another, and parties must set those out as best they can before an election. Clegg believes he is doing that: he talked of liberalism, warned of savage cuts, and promised to create a different kind of state – and the consequences can be traced everywhere in coalition policy.

Oppositionalist purists will rant against the compromises of power. Nobody knows better than the Lib Dems how easy that is to do. But they've taken the decision to stop copping out. The neurotic rage of those who still want to is entirely predictable